This is a tiny little project (about 1 gram of filament), which simply creates a refrigerator magnet out of a 10mm diameter x 3mm neodymium magnet in the shape of a popular type of thumbtack.
I bought 35 of these magnets from Amazon (US) here https://www.amazon.com/Refrigerator-JACK-CHLOE-Stainless-Approximately/dp/B01MFHNHNW/ for about $9.00.
The final parts are about 18.5mm tall and make a nice size to hold your notes and such to any metallic surface.
The design requires you to pause printing once the layer 3.5mm off the build plate completes. If you're using a Prusa Printer, check out the gcode files here (https://www.prusaprinters.org/prints/1817-push-pin-fridge-magnet) which will automatically pause the printer at the correct time and wait for you to insert your magnet.
I've included the Fusion 360 archive for this project so you can make your own with other magnet sizes. Also, I've posted a video here of how it works/how it was made: https://youtu.be/lAp39FZbVRQ.